1. Field of Invention
The invention relates to a control apparatus for a multi-speed automatic transmission which is equipped with a manual shift mode that allows the speed ratio step to be shifted up or down manually by an operation of a driver of the vehicle.
2. Description of Related Art
There is a multi-speed automatic transmission of a vehicle that is able to switch between an automatic shift mode in which an appropriate speed ratio step is automatically set on the basis of the state of travel of the vehicle and a manual shift mode that allows the speed ratio step to be shifted up or down manually by an operation of a driver of the vehicle. During the manual shift mode of this multi-speed automatic transmission, the speed ratio step of the multi-speed automatic transmission is set, in principle, according to the operation of the driver. However, this automatic transmission is designed so that the downshift operation by the driver is cancelled in the case where the engine rotation speed becomes excessively high due to a rise in the turbine rotation speed after the downshift. For example, Japanese Patent Application Publication No. 2008-111510 (JP-A-2008-111510) discloses a control apparatus for a multi-speed automatic transmission that cancels a driver's downshift operation when the vehicle speed exceeds a downshift-permitting vehicle speed that is set at a vehicle speed that is lower than an automatic upshift vehicle speed. Incidentally, the determination regarding the permission of a downshift is based on not only the vehicle speed, but also the engine rotation speed, or the turbine rotation speed of the multi-speed automatic transmission.
Besides, a control apparatus for a multi-speed automatic transmission which performs a control of increasing a downshift permission criterion value with regard to the vehicle speed, the engine rotation speed, or the turbine rotation speed of the multi-speed automatic transmission, is also known. In this control apparatus, the downshift permission criterion value set according to the speed ratio step brought about by a downshift is corrected with a speed-value-increasing correction amount that is computed according to the situation of travel of the vehicle at the time of start of the downshift. In this manner, the control apparatus variably sets the downshift permission criterion value according to the situation of travel of the vehicle. When the depression of the accelerator pedal is small and therefore the rising of the engine rotation speed before a downshift is completed is slow, the downshift permission criterion value is adjusted to a higher speed value (a higher vehicle speed value, a higher rotation speed value), so as to reduce the cancellation of a manual downshift. As a control apparatus for a multi-speed automatic transmission of this kind, Japanese Patent Application Publication No. 2006-258125 (JP-A-2006-258125), for example, discloses a control apparatus that cancels the downshift operation of a driver when the engine rotation speed exceeds a downshift permission criterion value, and that adjusts the downshift permission criterion value to a higher speed value when the degree of deceleration of the vehicle is great.
By the way, a downshift over a plurality of steps, that is, a downshift that passes at least one speed ratio step, may sometimes be required depending on a driver's operation. Then, depending on the numbers of steps of downshifts, the gear ratios of the multi-speed automatic transmission achieved by the downshifts vary, and therefore the amounts of change in the turbine rotation speed achieved by the downshifts also vary even if the accelerator operation amount is fixed. In the foregoing control apparatuses for multi-speed automatic transmissions according to the related art, however, the speed-value-increasing correction amount is computed in a uniform manner regardless of the number of steps of a downshift. Therefore, with the foregoing control apparatuses, it sometimes happens that at the time of a downshift that occurs over a plurality of steps, the correction of the downshift permission criterion value becomes excessive, so that after the downshift, an upshift immediately needs to be performed, or after the downshift, excessively high engine rotation speed occurs.